1. Field of the Invention
The present invention relates to an image display device having a memory property and to be driven according to an electrophoretic display method and more particularly to the image display device having the memory property that can be suitably used for electronic paper display such as electronic books, electronic newspaper and the like.
2. Description of the Related Art
As a display device capable of doing a deed of “reading” without a stress, an electronic paper display device referred to as an electronic book, electronic newspaper and the like is now under development. Since it is necessary that that the electronic paper display of this kind is thin, light weight, hard to crack, and low in power consumption, its construction by using a display element having a memory property is desirable.
As a display element to be used in a device having a memory property, conventionally, an electrophoretic display element or cholesteric liquid crystal or the like is known, however, in recent years, electrophoretic display elements of two or more kinds are attracting attention. In this specification, the electrophoretic display element conceptually contains a device such as a quick-response liquid powder element that can achieve displaying by causing electrically charged particles to move.
First as related arts, an electrophoretic display device of the type that displays white and black colors by active matrix driving method is described. The electrophoretic display device is so configured that a TFT (Thin Film Transistor) glass substrate, electrophoretic display element film, and facing substrate are stacked in layers in this order. On the TFT glass substrate, TFTs arranged in a matrix manner, a pixel electrode connected to each TFT, gate lines driving TFTs, and data lines are mounted.
The electrophoretic display device is configured in a manner in which micro capsules being about 40 μm in size spread in a polymer binder. A solvent is injected into an inner portion of each of the micro capsules and, in the solvent, two kinds of positively and negatively charged nano-particles, that is, a white pigment made up of negatively charged titanium dioxide particles and a black pigment made up of positively charged carbon particles are hermetically confined within a dispersed and floated state. Moreover, on the facing substrate, a facing electrode (also called a common electrode) to provide a reference potential is formed.
The electrophoretic display device is operated by applying a voltage corresponding to pixel data between the pixel electrode and facing electrode and by moving the white and black pigments up and down. That is, when a positive voltage is applied to the pixel electrode while the positively charged black pigment is attracted by the facing electrode and, therefore, by using the facing electrode side as its display, black is displayed on the screen.
Further, when a negative voltage is applied to the pixel electrode, the positively charged black pigment are attracted by the pixel electrode while the negatively charged white pigment are attracted by the facing electrode and, as a result, white is displayed on the screen.
Next, when an image display is to be changed from white to black, a positive signal voltage is applied to the pixel electrode and, when the image display is changed from black to white, a negative signal voltage is applied to the pixel electrode, and when a current image display is to be maintained, that is, the white display or the black display is maintained, due to a memory property, 0V is applied. Thus, by comparing the current screen (previous screen) with a next screen (screen to be renewed), a signal to be applied is determined.
Moreover, an electrophoretic display device that can display colors in order of a unit pixel without losing a color feeling in white and black as in the case of paper and without using a color filter is being developed. For example, in Patent Reference 1 (Japanese Patent No. 4049202), an electrophoretic color display device is disclosed which is made up of an electrophoretic layer containing electrophoretic particles of the same polarity having these colors each being different from one another (for example, cyan (C), magenta (M), and yellow (Y) and having a white (W) supporting body to support the electrophoretic particles.
Each of the electrophoretic particles providing the three colors has a threshold value voltage to initiate an electrophoresis (electrophoresis initiating voltage) set so as to be different from one another. In the color electrophoretic display device disclosed in the Patent Reference 1, by utilizing a difference in the threshold voltage (absolute value) and by controlling a voltage to be applied to each electrophoretic particle, one cell can display cyan (C), magenta (M), and yellow (Y) in addition to white (W) and black (K), and second color and third color of these CMY colors.
Further, another color electrophoretic display device is disclosed in Patent Reference 2 (Japanese Patent No. 4385438) which uses an electrophoretic display device film on which various micro capsules spread in a layer state. A black first charged particle having charge of a first polarity, second charged particles R, G, B in red (R), green (G), and blue (B) colors having charge of a second polarity, and liquid dispersion medium to disperse these particles in a manner in which an electrophoresis can occur are enclosed hermetically in the above micro capsules.
Here, the second charged particles R, G, B have charged amounts different from one another and each particle has a threshold value voltage to initiate an electrophoresis being different from one another and is hermetically enclosed in a separate microcapsule being different from one another.
In the color electrophoretic display device disclosed in Patent Reference 2, by using a difference in a threshold value voltage (absolute value), a voltage to be applied to each electrophoretic particle is controlled and, therefore, each cell, without a color filter as in the case of the Patent Reference 1, can display second and third colors of RGB.
In the Patent Reference 3 (Japanese Patent Application Laid-open No. 2009-47737), a color electrophoretic display element is disclosed which uses electrophoretic particles having not only 3 colors including cyan (C), magenta (M) and yellow (Y) but also a color of black (K), 4 colors in total.
Thus, according to technologies disclosed in the Patent Reference 1, 2, and 3, the color display is made possible by three threshold values provided by each of the charged particles C, M, Y (or R, G, B). Display operations of the color electrophoretic display device disclosed in the Patent Reference 1 is described by referring to FIGS. 32 and 33. The threshold value voltages Vth(c), Vth(m), and Vth(y) for respectively each of charged particles C, M, Y are set so as to satisfy the relationship of |Vth(c)|<|Vth(m)|<|Vth(y)|. Each of applied voltages V1, V2, and V3 is set so as to satisfy the relationship of |Vth(c)|<|V3|<|Vth(m)|, |Vth(m)|<|V2|<|Vth(y)|, |Vth(y)|<|V1|.
FIGS. 32 and 33 show hysteresis curves of charged particles C, M, and Y, representing a relation between a threshold voltage and a relative color density. Moreover, in FIGS. 32 and 33, for simplification, so that a gradient of each hysteresis Y, nY, M, nM, C and nC is constant, the time required for movement of Y, M, C from a rear to a display surface is set to be different from one another.
In FIG. 32, an initial (previous) screen is supposed to be white (W). While white (W) is being displayed, if V3 (=10V) is applied, a cyan color electrophoretic particle C moves to a display surface side and, therefore, cyan (C) is displayed on a next screen. While white (W) is being displayed, if V2 (=15V) is applied, cyan (C) and magenta (M) color electrophoretic particles move to a display surface side, blue (B) is displayed.
While white is being displayed, if V1 (=30V) is applied, cyan (C), magenta (M), and yellow (Y) color electrophoretic particles C, M, and Y move to the display surface side and, as a result, black (K) is displayed. While white (W) is being displayed, if a negative voltage is applied, no color particle exists and white (W) is still being displayed.
Next, a previous screen is made black (K). While black is being displayed, if −V3 (=−10V) is applied, a cyan color electrophoretic particle C moves to a rear substrate side and the magenta (M) and yellow (Y) electrophoretic particles M and Y are left and, therefore, red (R) is displayed on a next screen.
While black is being displayed, if −V2 (=−15V) is applied, cyan and magenta color electrophoretic particles C and M move to the rear substrate side and yellow electrophoretic particle Y is left on the display surface side and, as a result, yellow (Y) is displayed. While black is being displayed, if −V1 (=−30V) is applied, cyan (C), magenta (M) and yellow (Y) color electrophoretic particles C, M, Y move to a rear substrate side and white (W) is displayed.
In order to display a magenta (M) color, as shown in FIG. 33, while white is being displayed, V2 (=15V) is applied to move the cyan (C) and magenta (M) color electrophoretic particles C and M to the display surface side and an intermediate transition state having a blue (B) color is allowed to occur.
While a state is in the intermediate transition state, −V3 (=−10V) is applied to move the cyan (C) color electrophoretic particle C to the rear side and, then, magenta (M) is displayed (see Table 12). Moreover, in order to display a green (G) color, as shown in FIG. 32, while black is being displayed, −V2 (=−15V) is applied to move cyan (C) and magenta (M) electrophoretic particles C and M to the rear side and an intermediate transition state having a yellow (Y) color is allowed to occur. While a state is being in the intermediate transition state, V3 (=10V) is applied to move the cyan (C) color electrophoretic particle C to the display surface side to display a green (G) color (see Table 12).
Thus, when a previous screen is in a white (W) state, as shown in Table 12, the state of a primary color to which a direct transition is possible is cyan (C), blue (B), and black (K). Similarly, as shown in Table 12, through black intermediate transition I, red (R) or yellow (Y) is displayed. Through blue (B) intermediate transition state I, magenta (M) is displayed and through black (K) and yellow (Y) intermediate transition state I, II, green (G) is displayed (see Table 12).
TABLE 12IntermediateIntermediatePrevious ScreenTransition ITransition IIRenewed ScreenW——WW——KW——CWBMWKYWKRWKYGW——B
As described above, in the electrophoretic display device disclosed in the Patent Reference I which uses a difference in a threshold voltage, from a ground state, primary colors being red (R), green (G), blue (B), cyan (C), magenta (M), yellow (Y), white (W) and black (K) can be displayed.
This is true for the electrophoretic display device disclosed in the Patent Reference 2 to 3, however, the display devices described in the Patent References have defects that, at time of renewal from a previous screen to a next screen, the renewal is realized through an intermediate transition of one or more primary colors (relative color density being 1) and, as a result, discomfort “flickering” caused by great and rapid changes in luminance and color density during the renewal processes.
Additionally, displaying of given display color La*b* including an intermediate and/or gray level displaying using three colors charged particles C, M, Y on a same pixel electrode is very complicate and this problem is not yet solved by the technologies in the Patent Reference 1 to 3.